Means and method for relaying frequency modulated signals



Patented Dec. 2, 1941' MEANS METHOD FOR RELAYING FRE- QUENCY MODULATEDSIGNALS Edwin H. Armstrong, New York, N. Y. Application January 12,1s4o,-seriamo. 313,496

10 Claims.

wide band frequency modulation system described in my U. S. Patent#1,941,069 which have been assigned frequencies in the range of 42.5 to43.5 megacycles. The various channels assigned are 200 k. c. in width,and hence the frequency difference between the frequency of the signalwhich is to be received and the frequency which is to,

be retransmitted may be very small, requiring therefore highly selectivereceivers. In addition to the high selectivity of the receiver, it isusually necessary to adopt the further precaution of placing thereceiver at some distance from the transmitting antenna to preventvarious forms of interference from the very high signal levelencountered in the immediate field of the transmitter.

There are two types of relay transmitting systems. In one the receivedsignal is detected and converted into its original form, and thiscurrent is used to modulate the second transmitter. In the other thereceived radio frequency current is heterodyned to the frequency to betransmitted, amplified up and transmitted without ever being convertedinto the audio or other signaling current. In each type of relayingcertain difliculties are encountered. In the first type reception wouldbe carried out with a superheterodyne which can be readily designed togive the necessary selectivity to prevent interference from thetransmitter, but which suffers from the fact that in the detection atthe receiver and the remodulation of the local transmitter a certainamount of distortion is encountered. This. while hardly detectable in asingle relay, eventually places a limit on the number of retransmissionswhich could be undertaken. In the second type reception would beaccomplished without converting to audio, and the frequency to betransmitted would be produced by adding to, or subtracting from,

the incoming frequency a locally produced current which might be of theorder of 400 k. c. The desired frequency component would be selected outand transmitted and in this way the distortion incident to detection anmodulation would be avoided. The circuits required to select 65 thedesired frequency for retransmission and the circuits required toprotect the receiver from the effects of the transmitter would ofnecessity have to be extremely selective and consequently critical toadjust and maintain.

The present invention has for its object the provision of means wherebythe difficulties of both types are overcome and the advantage of bothretained. In accordance with the present method the incoming signal isheterodyned down to some relatively low intermediate frequency at whichthe desired selectivity may be obtained, converted to a frequency whichdiffers from the initial intermediate frequency by an amount equal tothe difference between the received frequency and the frequency to beradiated and heterodyned up by the same amount by which it was initiallyheterodyned down. In order that changes in the frequency of theheterodyning currents used in the conversion of the incoming frequencydown to its initial intermediate frequency and in the conversion of thesecond intermediate frequency up to the outgoing frequency may notproduce undesired drift therein, both of these heterodyning currents areobtained from the same oscillating source so that any drift in thefrequency of one current occurs simultaneously in the other. Theoperation of the system may be more exactly understood from thefollowing explanation.

Referring now to the figures which form a part of this specification,Figs. I and II represent the complete relay system in which theapparatus shown in Fig. I will usually be separated from the apparatusof Fig. II by a distance of some hundreds of feet, Fig. 11 being locatedat the transmitter. While this is not essential under all conditions,where weak signals are to be relayed it is desirable and the arrangementof apparatus required for separated locations is, therefore, depicted.

Referring now to the arrangement of Figs. I and H, it has been assumedthat a frequency of 42.8 megacycles isto be received and retransmittedas 43.4 megacycles. I represents the receiving antenna connected throughfilter 2 to a balanced converter 3. This converter is supplied with aheterodyning current of 41.8 megacycles as shown. 4 represents a filterhaving a pass band sufilcientlywide to pass the .90lL- 1100 k. c.current produced by the interaction of the incoming 42.8 megacyclessignal with a 100 k. c. deviation, and the fixed frequency of 41.8megacycles. 5, 6, I and 8 represent selective amplifiers, 9 a limiterfor the 1000 k. c. current, It! a filter, ii an amplifier, and 12 asecond filter connecting to a line I3. It has been assumed that adistance of several hundred feet separated the arrangements of Figs. Iand II. The line l3 which terminates at l8 in Fig. II should preferablybe a concentric or balanced line. Referring now back to the heterodyningcurrent of 41.8 megacycles, this is initially derived from a currentsupplied over a line II from an oscillator at the other location at10.45 megaeycles through an amplifier l5, through a doubler it whichraises it to 20.9 megacycles and a second doubler H which supplies 41.8megacycles current to the amplifier l8.

Referring now to Fig. II, l9 represents the end of the line carrying the1000 k. c. intermediate frequency current, 20 a band pass fliter, 2| anamplifier, and 22 a balanced converter. A local oscillator 23,preferably Xtal controlled, excites an amplifier 24 which supplies theconverter 22. The output of the converter is supplied through a bandpass filter 25 having a pass band of 1500-1700 k. c. to an amplifier 26whose output is fed into a balanced converter 21. The heterodyningcurrent for this converter is obtained from a 10.45 megacycle oscillator28 through an amplifier 29. 34 represents a band pass filter of 200 k.0. width and a midfrequency point of 12.05 megacycles. 35 represents anamplifier for this frequency, and 36 a second converter. This converteris supplied with a heterodyning current of 31.35 megacycles derived fromthe 10.45 megacycles source and raised to the 31.35 megacycles value bythe tripler 32. 33 represents an amplifier for this frequency. 31represents an amplifier for 43.4 megacycle output current of theconverter 35, 38 the power amplifier, and 39 the antenna. Referring backto the oscillator 28, an amplifier 30 is excited by current derivedtherefrom, and supplied to a line 3| which terminates at H in Fig. 1.From current delivered by this line the frequency multiplication systeml-l8 produces the initial heterodyning current.

It will be observed from the foregoing explanation that if the frequencyof the first station is maintained on its exact frequency that thesecond station must of necessity likewise be on its correct frequency,since the only element in the relay whose frequency stability in any wayhas a bearing on the stability of the frequency of the second station isthe 600 k. c. oscillator. Since this is such a small fraction of thevalues which we are considering, any

variation which could normally occur in it is bound to be numericallyinsignificant in the final result. As previously explained, any drift inthe frequency of the 10,450 k. c. oscillator is of no importance sinceit is self-cancelling.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be confined to the embodimentsshown, but what I desire to cover by letters Patent is set forth in theappended claims.

I claim:

1. The method of relaying frequency modulated signals ,on slightlydifferent wave lengths which consists in receiving the wave of the firstlength, heterodyning the received wave down to a low intermediatefrequencyby a heterodyning current which is a higher harmonic of a givenlow frequency oscillator, converting the low intermediate frequency toanother frequency whose difference therefrom is equal to the frequencydifference between the received and transmitted waves, raising thelast-named frequency to the frequency to be transmitted by adding to itsuccessively the frequency of the fundamental and an appropriateharmonic of said given low frequency heterodyning oscillator andtransmitting currents of the resulting frequency.

2. The method of relaying frequency modulated signals on slightlydifferent wave lengths which consists in receiving the wave of the firstlength, heterodyning the received wave down to a low intermediatefrequency by a heterodyning current which is a harmonic of a given lowfrequency oscillator, amplifying the currents of low intermediatefrequency, converting the amplified currents of low intermediatefrequency to an other frequency whose difference therefrom is equal tothe frequency difference between the received and transmitted waves,raising the lastnamed frequency to the frequency to be transmitted byadding to said last named frequency energy controlled by said given lowfrequency heterodyning oscillator and transmitting currents of theresulting frequency.

3. The method of relaying frequency modulated signals on slightlydifferent wave lengths which consists in receiving the wave of the firstlength, heterodyning the received wave down to a low intermediatefrequency by a heterodyning current which is a harmonic of a lowfrequency oscillator generating a substantially constant frequency,amplifying the currents of low intermediate frequency, converting theamplified currents of low intermediate frequency to a second lowfrequency whose difference therefrom is equal to the frequencydifference between the received and transmitted waves, filtering andamplifying the currents of the second low frequency, converting theamplified currents of the second low frequency to the frequency to betransmitted by adding to said last named frequency energy controlled bysaid low frequency heterodyning oscillator and transmitting currents ofthe resulting frequency.

4. Means for relaying frequency modulated signais on slightly differentwave lengths comprising, in combination, a converting device, means forenergizing said converting device by the received wave of the firstlength, a first low frequency oscillator, frequency multiplying meanscoupled to said oscillator for generating a harmonic of the oscillatorfrequency, means conmeeting said frequency multiplying means to theconverting device whereby currents are generated therein of a lowintermediate frequency, a second converting device having its inputconnected to the output of the first converting device, a second lowfrequency oscillator connected to the input of the second convertingdevice and arranged to generate a frequency equal to the frequencydifference between the received and transmitted waves, means coupled tothe output of the second converting device for increasing the frequencyof the currents therein to the frequency to be transmitted, said coupledmeans comprising means for generating a harmonic frequency of the firstlow frequency oscillator, and means for transmitting currents of theresulting frequency.

5. Means for relaying frequency modulated signals on slightly differentwave lengths comprising, in combination, a converting device, means forenergizing said converting device by the received wave of the firstlength, a first oscillator. frequency multiplying means coupled to saidoscillator for generating a harmonic of the oscillator frequency, meansconnecting said frequency multiplying means to the converting devicewhereby currents are generated therein of a low intermediate frequency,a second converting device having its input connected to the output ofthe first converting device, a low frequency oscillator connected to theinput of the second converting device and arranged to generate afrequency equal to the frequency difference between the received andtransmitted waves, a third converting device connected to the secondconlength, generating a fundamental frequency and a higher harmonic ofsuch fundamental frequency, heterodyning the received wave down to afirst intermediate frequency by a heterodyningcurrent of said higherharmonic frequency, generating a radio frequencywhich is equal to thefrequency diflerence between the received and transmitted waves,increasing said first intermediate frequency to a substantially higherinverting device, means for applying to the input of the thirdconverting device a voltage component having the fundamental frequencyof the first oscillator and also a voltage component which is aharmonic' of the fundamental frequency, amplifying; means connected tothe output of the third-converting device, and means for transmittingthe amplified currents.

- 6. The method-of relaying frequency modulated radio signals onslightly different wave lengths which consists in receiving the wave ofthe first'length, heterodyning the received wave down to a lowintermediate frequency by a heterodyning'current which is a harmonicof'a given lowfrequency oscillator, ,raising the lowintermediatefrequency to the frequency to be transmitted by adding thereto aplurality of frequencies, one frequency of said plurality being ,equalto thedifference between the frequencies of the received and transmittedwaves and the remaining frequencies of said plurality being controlledby the said low frequency oscillator. A

'7. Means for relaying frequency modulated radio signals on slightlydifferent wave lengths comprising, in combination, means for receiv ingthe wave on the first length, a lowfrequency oscillator, means forgenerating harmonic fretermediate frequency by adding thereto saidfundamental and said radio frequencies, increasing said higherintermediate frequency to the frequency to be transmitted by addingthereto at least a higher harmonic of said fundamental frequen y, andtransmitting currents of the resulting frequency.

9. A system for retransmitting a frequency modulated wave at a frequencywhich difiers slightly from the frequency at which it comes in, saidsystem comprising means for heterodyning the incoming frequency down toa desired low intermediate frequency, means for heterodyning thelow'intermediate frequency current upto the desired frequency ofretransmission, a

' quency which differs slightly from the frequency quencies of saidoscillator, means connected to said receiving means and to said harmonicgenerating means for heterodyning the received wave down to a lowintermediate frequency, means for generating a frequency which is equalto the difference in frequencies between the received and transmittedwaves and frequency converting means connected to the output of saidlast named means, to the output of said heterodyning means and to anoutput of said harmonic generating means for raising the lowintermediate frequency to the frequency to be transmitted.

8. The method of relaying frequency modulated signals on slightlydifferent wave len ths which consists in receiving the wave of the firstat which it comes in, which consists in generating a heterodyningfrequency, generatinga difference frequency equal to the difference be-"tween the frequencies of the received and transmitted waves,heterodyning the incoming frequency by said heterodyning frequency downto a desired low intermediate frequency, heterodyning the lowintermediate frequency current up to the desired transmitting frequencyby means of a plurality of frequencies whose algebraic sum is equal tothe aggregate of said heterodyning and said differencefrequencies, andeliminating from the outgoing wave the effects of, amplitude variationsin said incoming wave. I

EDWIN- H. ARMS'I'RQNG.

